According to a speech made this week by the deputy prime minister, there are "few more powerful illustrations of just how divided our society can be" than the continuing Up series of television documentaries, which began in 1964 with Seven Up! and has revisited its participants once every seven years since. "What hits you hardest", Nick Clegg went on, "is that in the half-century since the series began, little has changed. Our society is still too closed, too static. A society that still says where you are born, and who you are born to, matters for the rest of your life."

Up to a point. What hit me hardest about Monday's episode of 56 Up, the second of three, was its disappointing compression: too much was squeezed into too little time, with too much left unsaid. Britain may have some of the lowest levels of social mobility in the developed world, as measured by OECD figures that show an individual's earnings in the UK are more likely to reflect his or her father's than in than any other country, but on the evidence of the Up series so far, it would be hard to conclude that Britain's class divisions are set in concrete. That was the point makers of the original programme hoped to make when they dispatched two researchers across England to find seven-year-olds who might vividly represent class difference. But as the series went on, that sociological and political intention got lost in the interestingness of 14 unfolding lives.

"Why do we bring these children together?" says a long-ago voice on the original commentary. "Because we want to get a glimpse of England in the year 2000. The union leader and the business executive of the year 2000 are now seven years old." But nobody in the series has become an executive or a union leader, and the notion that these two categories represent opposing ends of the social spectrum looks hopelessly antique: Peter Sellers versus Dennis Price in I'm All Right Jack; the overall versus the suit; the canteen versus the staff restaurant. And all this imagery dependent on manufacturing processes that a Granada producer in Manchester could sniff if he opened a window.

The world turned out to have less predictable patterns. Of the three working-class girls from London, Sue is a university administrator and Lynn a librarian, while their friend Jackie, who has rheumatoid arthritis, can find no paid work in her adopted town of Motherwell. Tony, another Londoner, became first a jockey and then a taxi driver and now has homes in Essex and Spain. Paul, one of two boys first seen in a children's home, helps his wife run an old people's home in Australia. The other boy, Symon, drives a forklift truck.

As for the middle-class contingent, we find that Nick, the Yorkshire farmer's son, is a professor of electrical engineering in Wisconsin, while two boys from the Liverpool suburbs, Neil and Peter, are respectively a civil servant and a Lib Dem councillor in Cumbria, living frugally on his councillor's allowance. That leaves a group of five, whose voices and private schooling marked them out as upper middle class, or perhaps (bring on Henry Higgins) the lower reaches of the upper class. Suzy married a prosperous lawyer, Rupert, and so far as we can tell, lives happily with her family in a house with a tennis court. One boy dropped out of the series after 1977. The remaining three, to be seen in Monday's third episode, went to Oxbridge. Two became lawyers. The third, Bruce, taught in Bangladesh and east London before he joined the staff of a public school in Hertfordshire.

Perhaps because these films began in a more courteous time and quickly became studies of personal history rather than an inquiry into social class, they contain few statistics. We don't know what people earn or the worth of their houses, if they own one. It's reasonable to suppose that all those who got divorced would be richer if they'd stayed married, and that Jackie, anxious about her disability benefit, is the least well off. But what about the social mobility that in Clegg's words is "the central social preoccupation of the coalition government"? It isn't entirely absent. From the outside, it looks as if taxi driver Tony and university administrator Sue have climbed furthest, while middle-class Neil, who had a breakdown and spent time on the road, fell lowest. At least in financial terms, the traffic between middle and working class has been reasonably down as well as up.

Those who look most impregnable – most immune to downward movement – come from the highest layer. What puts them there? A certain kind of education – Clegg's kind – is at least part of the answer. Is there room for more on top? Clegg believes so, and not only more but better and brighter. Through targeted educational spending and monitoring instruments, such as the amazingly named Social Mobility Sector Transparency Board, Clegg hopes to send more children from poor families up the ladder to the top universities (at Oxbridge, only one in 100 students have taken free school meals, compared to the one in five pupils who take them at school). Achieving greater social mobility, we need to understand, has little to do with increasing social equality. A conference held by the Sutton Trust, which Clegg was addressing, unveiled research that showed how poor children in Australia and Canada stand a better chance of moving up than those in the UK and US, though the gaps between rich and poor are broadly similar in all four countries.

Phew! No inconvenient need to worry about egalitarianism! Or, as Clegg put it: "Of course, reducing inequality is a good and laudable aim. But unfortunately, it's not the straightforward route to social mobility that its proponents suggest. In many ways, I wish it was. Life would be much simpler. Our goal would be clear: redistribution of income would do the job." (The "I wish it was" is delicious.)

Sir Alec Douglas-Home was prime minister when Seven Up! was first broadcast. An old Etonian, he had disclaimed his earldom the previous year to fight the by-election that got him into the Commons. Harold Wilson mocked him as an "elegant anachronism". Still, the income ratios between rich and poor were closer then, and if social mobility was what you wanted, grammar schools were there to provide it. You went nervously into a classroom one day – so much depended on the outcome – and sat the 11-plus (in Scotland "the quali" or qualifying exam) and if you had enough right answers, you joined the academic elect. It was divisive and hideously unfair, but almost certainly less so than any future selection for advancement towards the holy grail of the Russell Group aided by the likes of the Social Mobility Sector Transparency Board. Social mobility being in such demand, the puzzle is the coalition's refusal to reintroduce grammar schools to every corner of the country. Their blazers could have badges with the motto "Liberty, Mobility, No Equality".